/Engine/Source/Runtime/UMG/Public/Blueprint/WidgetTree.h
Determines the child index of the given ParentWidget that the given ChildWidget ultimately occupies, accounting for nesting
The child slot index within ParentWidget that ChildWidget ultimately occupies (INDEX_NONE if ChildWidget is not within ParentWidget at any level)
static int32 FindChildIndex ( const [UPanelWidget](API\Runtime\UMG\Components\UPanelWidget) * ParentWidget, const [UWidget](API\Runtime\UMG\Components\UWidget) * ChildWidget )